The sediment release and cycling of CH4in SO2−4– rich hypereutrophic Onondaga Lake, New York, was assessed for the summer/fall interval of 1989 through a program of collection and analysis of ebullitive gas and monitoring of the lake's water column for dissolved CH4. Approximately 1×107mol of CH4were released from the lake's sediments, at an average rate of 16 mmol/m2/d, from mid-June to mid-September; two-thirds was dissolved, one-third was in the gas phase. About 75% of the ebullitive release of CH4, or 25% of the total release, was lost from the lake to the atmosphere. Strong temporal variations in ebullitive flux were observed. Oxidation of entrained hypolimnetic accumulations of CH4contributed importantly to the major depletion of O2and highly undersaturated conditions observed for the lake's upper layers during the fall mixing period. This water quality problem is a manifestation of the cultural eutrophication of the lake.